Increased expression of cyclooxygenase-2 in the renal cortex of human prorenin receptor gene-transgenic rats

Increased macula densa cyclooxygenase-2 (COX-2) is observed in diabetic rats and may contribute to hyperfiltration states. However, the signals mediating increased COX-2 expression in diabetic rats remain undetermined. We recently found that non-proteolytic activation of prorenin by site-specific binding proteins, such as prorenin receptor, plays a pivotal role in the development of diabetic nephropathy. The present study was designed to determine the contribution of prorenin receptor to renal cortical COX-2 expression. The COX-2 mRNA and protein levels of six 4-week-old male wild-type rats and six human prorenin receptor gene-transgenic (hProRenRcTg) rats were measured by real-time polymerase chain reaction methods, Western blotting, and immunohistochemistry, and compared. There were no differences between the two groups in arterial pressure measured by telemetry, urinary sodium excretion, or renal levels of rat prorenin receptor mRNA. The renal cortical COX-2 mRNA levels of the hProRenRcTg rats were significantly higher than those of the wild-type rats, and the renal cortical COX-2 protein levels were also higher in hProRenRcTg rats than in the wild-type rats. Immunohistochemical analysis revealed that COX-2 immunostaining was predominantly present in the macula densa cells, and significantly more COX-2-positive cells were present in the hProRenRcTg rats than in the wild-type rats. In addition, COX-2 inhibition with NS398 significantly decreased renal cortical blood flow in the hProRenRcTg rats but not in the wild-type rats. These results strongly suggest that human prorenin receptor directly or indirectly contributes to the regulation of renal cortical COX-2 expression.

Metalloprotease-dependent ErbB ligand shedding in mediating EGFR transactivation and vascular remodelling

AngII (angiotensin II) and its G-protein-coupled AT(1) receptor play critical roles in mediating cardiovascular diseases such as hypertension, atherosclerosis and restenosis after vascular injury. It is widely believed that AngII promotes these diseases by inducing vascular remodelling that involves hypertrophy, hyperplasia and migration of VSMCs (vascular smooth muscle cells). We have shown that transactivation of an ErbB family receptor, EGFR (epidermal growth factor receptor; ErbB1), is essential for VSMC hypertrophy and migration induced by AngII. However, the precise signal transduction mechanism by which AngII transactivates EGFR/ErbB1 and whether other ErbBs are also required for AngII function remains unclear. Recent studies suggest an involvement of a metalloprotease-dependent ErbB family ligand production in the transactivation. Here, we will discuss the roles and mechanisms of AngII/AT(1) receptor in promoting ErbB receptors transactivation in VSMCs. Further elucidation of this ErbB activation machinery not only will give us a better understanding of the critical molecular mechanism underlying vascular remodelling stimulated by AngII, but will also contribute to development of novel treatment strategies for cardiovascular diseases.

Dual effects of angiotensin II type 2 receptor on cardiovascular hypertrophy

Roles of angiotensin II (Ang II) in the regulation of cardiovascular system under normal and pathological condition have been well documented. Although two major subtypes of Ang II receptors, AT(1) and AT(2), are found in various proportions, the role and signaling mechanisms of AT(2) in the control of hypertrophic responses of cardiac ventricle and vasculature are not clear. Although earlier reports indicated that AT(2)'s functions are essentially growth suppression, an increasing number of recent reports indicate that AT(2) in cardiovascular tissues are often growth promoting. In some tissues AT(1) and AT(2) seem to share a common signaling pathways, at least in part. This review focuses on the accumulating evidence for the AT(2) function in the cardiovascular system.

Increased AT(1) receptors in adrenal gland of AT(2) receptor gene-disrupted mice

Angiotensin II (Ang II) AT(2) receptor-gene disrupted mice have increased systemic blood pressure and response to exogenous Angiotensin II. To clarify the mechanism of these changes, we studied adrenal AT(1) receptor expression and mRNA by receptor autoradiography and in situ hybridization in female AT(2) receptor-gene disrupted mice (agtr 2-/-) and wild-type controls (agtr 2+/+). We found high expression of AT(1) receptor binding and mRNA in adrenal zona glomerulosa of female wild-type mice. AT(2) receptors and mRNA were highly expressed in adrenal medulla of wild-type mice, but were not detected in zona glomerulosa. There was no AT(2) receptor binding or mRNA in adrenal glands of AT(2) receptor-gene disrupted mice. In these animals, AT(1) receptor binding and mRNA were increased in adrenal zona glomerulosa and AT(1) receptor mRNA was increased in the adrenal medulla when compared with wild-type animals.The present data support the hypothesis of an interaction or cross talk between AT(2) and AT(1) receptors in adrenal gland. The significant increase in AT(1) receptor expression in the absence of AT(2) receptor transcription may be partially responsible for the increased blood pressure and for the enhanced response to exogenously administered Angiotensin II in this model.

Overexpression of the human angiotensin II type 1 receptor in the rat heart augments load induced cardiac hypertrophy

Angiotensin II is known to stimulate cardiac hypertrophy and contractility. Most angiotensin II effects are mediated via membrane bound AT1 receptors. However, the role of myocardial AT1 receptors in cardiac hypertrophy and contractility is still rarely defined. To address the hypothesis that increased myocardial AT1 receptor density causes cardiac hypertrophy apart from high blood pressure we developed a transgenic rat model which expresses the human AT1 receptor under the control of the alpha-myosin heavy-chain promoter specifically in the myocardium. Expression was identified and quantified by northern blot analysis and radioligand binding assays, demonstrating overexpression of angiotensin II receptors in the transgenic rats up to 46 times the amount seen in nontransgenic rats. Coupling of the human AT1 receptor to rat G proteins and signal transduction cascade was verified by sensitivity to GTP-gamma-S and increased sensitivity of intracellular Ca2+ [Ca2+]i to angiotensin II in fluo-3 loaded transgenic cardiomyocytes. Transgenic rats exhibited normal cardiac growth and function under baseline conditions. Pronounced hypertrophic growth and contractile responses to angiotensin II, however, were noted in transgenic rats challenged by volume and pressure overload. In summary, we generated a new transgenic rat model that exhibits an upregulated myocardial AT1 receptor density and demonstrates augmented cardiac hypertrophy and contractile response to angiotensin II after volume and pressure overload, but not under baseline conditions.

The angiotensin II type 1 receptor and receptor-associated proteins

The mechanisms of regulation, activation and signal transduction of the angiotensin II (Ang II) type 1 (AT1) receptor have been studied extensively in the decade after its cloning. The AT1 receptor is a major component of the renin-angiotensin system (RAS). It mediates the classical biological actions of Ang II. Among the structures required for regulation and activation of the receptor, its carboxyl-terminal region plays crucial roles in receptor internalization, desensitization and phosphorylation. The mechanisms involved in heterotrimeric G-protein coupling to the receptor, activation of the downstream signaling pathway by G proteins and the Ang II signal transduction pathways leading to specific cellular responses are discussed. In addition, recent work on the identification and characterization of novel proteins associated with carboxyl-terminus of the AT1 receptor is presented. These novel proteins will advance our understanding of how the receptor is internalized and recycled as they provide molecular mechanisms for the activation and regulation of G-protein-coupled receptors.

Unique regulation of c-Jun N-terminal kinase by PYK2/CAK-beta in angiotensin II-stimulated vascular smooth muscle cells

Activation of tyrosine kinases is believed to play a central role in angiotensin II (AngII) signaling. Here, we have investigated whether a tyrosine kinase, PYK2, is functionally involved in AngII-induced c-Jun N-terminal kinase (JNK) activation in vascular smooth muscle cells (VSMCs). Adenovirus expressing PYK2 kinase-inactive mutant K457A or a tyrosine phosphorylation site mutant Y402F was transfected in VSMCs. AngII-induced JNK phosphorylation was markedly enhanced by K457A, whereas it was suppressed by Y402F. Protein synthesis induced by AngII was also enhanced by K457A and inhibited by Y402F. In this regard, K457A suppressed PYK2 kinase activation by AngII, whereas it enhanced AngII-induced PYK2 Tyr(402) phosphorylation. By contrast, Y402F inhibited PYK2 Tyr(402) phosphorylation, whereas it markedly enhanced AngII-induced PYK2 kinase activation. Thus, we conclude that PYK2 kinase activity negatively regulates JNK activation and protein synthesis, whereas Tyr(402) phosphorylation positively regulates these events in AngII-stimulated VSMCs, suggesting a unique role of PYK2 in mediating vascular remodeling.

Angiotensin II type 2 receptor is essential for left ventricular hypertrophy and cardiac fibrosis in chronic angiotensin II-induced hypertension

BACKGROUND

The roles of angiotensin II (Ang II) in the regulation of heart function under normal and pathological conditions have been well documented. Although 2 types of Ang II receptor (AT(1) and AT(2)) are found in various proportions, most studies have focused on AT(1)-coupled events. In the present study, we examined the hypothesis that signaling by AT(2) is important to the development of left ventricular hypertrophy and cardiac fibrosis by Ang II infusion in mice lacking the AT(2) gene (Agtr2-/Y).

METHODS AND RESULTS

Male Agtr2-/Y and age-matched wild-type (WT) mice were treated long-term with Ang II, infused at a rate of 4.2 ng. kg(-1). min(-1) for 3 weeks. Ang II elevated systolic blood pressure to comparable levels in Agtr2-/Y and WT mice. WT mice developed prominent concentric cardiac hypertrophy, prominent fibrosis, and impaired diastolic relaxation after Ang II infusion. In contrast, there was no cardiac hypertrophy in Agtr2-/Y mice. Agtr2-/Y mice, however, did not show signs of heart failure or impairment of ventricular relaxation and only negligible fibrosis after Ang II infusion. The absence of fibrosis may be a clue to the absence of impairment in ventricular relaxation and account for the normal left ventricular systolic and diastolic performances in Agtr2-/Y mice.

CONCLUSIONS

Chronic loss of AT(2) by gene targeting abolished left ventricular hypertrophy and cardiac fibrosis in mice with Ang II-induced hypertension.

3-Hydroxy-3-methylglutaryl-CoA reductase inhibitors block calcium-dependent tyrosine kinase Pyk2 activation by angiotensin II in vascular endothelial cells. involvement of geranylgeranylation of small G protein Rap1

We recently reported the calcium-dependent activation of tyrosine kinase Pyk2 by angiotensin II (Ang II) in pulmonary vein endothelial cells (PVEC). Since Pyk2 has no calcium binding domain, and neither Ca(2+) nor Ca(2+)/calmodulin directly activates Pyk2, it is not clear how Ca(2+) transduces the signal to activate Pyk2, a key tyrosine kinase, in the early events of Ang II signaling. In the present study, we investigated the mechanism of the calcium-dependent activation of Pyk2 in response to Ang II by using 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors and isoprenoid intermediates in PVEC. We have obtained substantial evidence indicating that Ang II activates Pyk2 through calcium-mediated activation of the geranylgeranylated small G protein Rap1 and the Rap1 association with Pyk2. Thus, the small G protein Rap1 is an intermediary signaling molecule linking Ang II-induced calcium signal to Pyk2 activation in PVEC. In addition, our results indicate that 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors, a class of cholesterol-lowering drugs, could interrupt Ang II signaling independent of cholesterol lowering in endothelial cells.

Changes of AT(2) receptor levels in the rat adrenal cortex and medulla induced by bilateral nephrectomy and its modulation by circulating ANG II

We studied regulation of the AT(2) receptor by investigating the effect of bilateral nephrectomy (bNX) in Sprague-Dawley rats. The expression of aldosterone synthase (CYP11B2) and AT(2) receptor mRNA was detected by nonradioactive in situ hybridization. AT(2) receptor mRNA was detected in cells of the first two or three subcapsular cell layers of the zona glomerulosa (ZG) and in the medulla of sham-operated animals. After bNX, the number and area of distribution of AT(2) receptor-positive cells increased in the ZG. This was associated with an enlargement of the steroidogenic active ZG and with reduced proliferation rate (sham 5.9 +/- 0.9%; bNX 2.4 +/- 0.2%; p<0.02). Infusion of angiotensin II (ANG II; 200 ng/kg/min SC for 56 hr) to bNX rats did not reverse the effect of nephrectomy on the distribution of AT(2) receptor expression, although mRNA levels per cell were reduced compared to NX alone. ANG II infusion decreased proliferation rate further (0.4 +/- 0.07%; p<0.001). In the adrenal medulla after bNX, decreased expression of the AT(2) receptor was associated with increased proliferation (2.6 +/- 0.2% vs 6.6 +/- 0.5%). These results demonstrate differential regulation of the AT(2) receptor in the adrenal gland and suggest that expression of the AT(2) receptor is involved in regulating proliferation and differentiation in the ZG and medulla. (J Histochem Cytochem 49:649-656, 2001)

Calcitonin receptor gene polymorphism in japanese women: correlation with body mass and bone mineral density

We have identified a polymorphism at position 1377 of the calcitonin receptor (CTR) gene which generates CC, CT, or TT genotype. In this study, the genotypes of the CTR and their relationship with the body height, the body weight, the bone mineral density (BMD), and osteocalcin levels were examined in 152 healthy Japanese women aged 16-43 years. The CTR genotypic frequencies in the Japanese population were 77.0% for CC, 20.4% for CT, and 2.6% for TT. The height, BMD, and osteocalcin levels were not significantly different among these three genotypes. The body weight adjusted for height in these three groups was significantly different in the order of TT, CT, and CC (P = 0.0454 by analysis of covariance). In combined analysis of the VDR genotype (B,b) and the CTR genotype (C,T), the body height was found to be significantly different between CCB and others (P = 0.0236). In addition, analysis of the CTR genotypic frequency using 64 blood samples from Japanese and 47 blood samples from Caucasians indicated that there was a significant difference between the two races (P < 0.0001). We found that C allele was predominant in the Japanese population, however, Caucasians have an almost equal ratio of the C and T. In conclusion, the CTR allele is one of the genetic factors regulating body weight in Japanese women.

Activation of MAPKs by angiotensin II in vascular smooth muscle cells. Metalloprotease-dependent EGF receptor activation is required for activation of ERK and p38 MAPK but not for JNK

In cultured vascular smooth muscle cells (VSMC), the vasculotrophic factor, angiotensin II (AngII) activates three major MAPKs via the G(q)-coupled AT1 receptor. Extracellular signal-regulated kinase (ERK) activation by AngII requires Ca(2+)-dependent "transactivation" of the EGF receptor that may involve a metalloprotease to stimulate processing of an EGF receptor ligand from its precursor. Whether EGF receptor transactivation also contributes to activation of other members of MAPKs such as p38MAPK and c-Jun N-terminal kinase (JNK) by AngII remains unclear. In the present study, we have examined the effects of a synthetic metalloprotease inhibitor BB2116, and the EGF receptor kinase inhibitor AG1478 on AngII-induced activation of MAPKs in cultured VSMC. BB2116 markedly inhibited ERK activation induced by AngII or the Ca(2+) ionophore without affecting the activation by EGF or PDGF. BB2116 as well as HB-EGF neutralizing antibody inhibited the EGF receptor transactivation by AngII, suggesting a critical role of HB-EGF in the metalloprotease-dependent EGF receptor transactivation. In addition to the ERK activation, activation of p38MAPK and JNK by AngII was inhibited by an AT1 receptor antagonist, RNH6270. and EGF markedly activate p38MAPK, whereas but not EGF markedly activates JNK, indicating the possible contribution of the EGF receptor transactivation to the p38MAPK activation. The findings that both BB2116 and AG1478 specifically inhibited activation of p38MAPK but not JNK by AngII support this hypothesis. From these data, we conclude that ERK and p38MAPK activation by AngII requires the metalloprotease-dependent EGF receptor transactivation, whereas the JNK activation is regulated without involvement of EGF receptor transactivation.

N-acetylcysteine inhibits angiotensin ii-mediated activation of extracellular signal-regulated kinase and epidermal growth factor receptor

Angiotensin II (Ang II) is known to stimulate reactive oxygen species (ROS) generation and epidermal growth factor (EGF) receptor transactivation to mediate growth-promoting signals such as extracellular signal-regulated kinase (ERK) in vascular smooth muscle cells (VSMCs). However, how ROS and EGF receptor interact to orchestrate these signals in VSMCs remains unclear. Here we found that an antioxidant, N-acetylcysteine, inhibited ERK activation and EGF receptor tyrosine phosphorylation induced by Ang II. Moreover, H(2)O(2) stimulates EGF receptor tyrosine phosphorylation and EGF receptor inhibitors attenuated H(2)O(2)-induced ERK activation. These data indicate that ROS mediate Ang II-induced EGF receptor transactivation, a critical mechanism for ERK-dependent growth in VSMCs.

Localization and characterization of a short isoform of the corticotropin-releasing factor receptor type 2alpha (CRF(2)alpha-tr) in the rat brain

We have recently isolated a cDNA encoding a short isoform of the corticotropin-releasing factor (CRF) receptor subtype, referred to as CRF(2)alpha-tr, from the rat amygdala. The present study determined the localization of the truncated receptor mRNA in the rat brain by in situ hybridization histochemistry. The results showed significant levels of hybridization in the lateral septum, central nucleus of the amygdala, cortico-amygdaloid nucleus, ventromedial nucleus of the hypothalamus (VMH), and frontal cortex. In the physiological study, antidepressive drugs increased the expression of CRF(2)alpha-tr mRNA and the total binding activity to CRF in the rat amygdala. These findings suggest that CRF(2)alpha-tr may regulate endogeneous CRF release in the amygdala.

Increased AT(1) receptor expression and mRNA in kidney glomeruli of AT(2) receptor gene-disrupted mice

The proposed feedback between angiotensin II AT(2) and AT(1) receptors prompted us to study AT(1) receptor expression in kidneys of male AT(2) receptor-gene disrupted mice (agtr2 -/y). In wild-type (agtr2 +/y) mice, AT(1) receptor binding and mRNA is abundant in glomeruli, and AT(1) receptor binding is also high in the inner stripe of the outer medulla. AT(2) receptors are scarce, primarily associated to cortical vascular structures. In agtr2 -/y mice, AT(1) receptor binding and mRNA were increased in the kidney glomeruli, and AT(1) receptor binding was higher in the rest of the cortex and outer stripe of the outer medulla, but not in its inner stripe, indicating different cellular regulation. Although AT(2) receptor expression is very low in male agtr 2 +/y mice, their gene disruption alters AT(1) receptor expression. AT(1) upregulation alone may explain the AT(2) gene-disrupted mice phenotype such as increased blood pressure, higher sensitivity to angiotensin II, and altered renal function. The indirect AT(1)/AT(2) receptor feedback could have clinical significance because AT(1) antagonists are widely used in medical practice.

Salutary role for angiotensin in partial urinary tract obstruction

BACKGROUND

In the neonatal period, angiotensin II (Ang II) is up-regulated and induces a timely development of the renal pelvis and ureteral peristalsis, thereby protecting the kidney from hydronephrosis. We tested the possibility that in adulthood, Ang II may act salutarily on the kidney structure during partial urinary tract obstruction by inducing adaptive changes in the peristaltic machinery.

METHODS

Adult male Sprague-Dawley rats were subjected to partial unilateral ureteral obstruction (UUO) and divided into two groups, that is, those treated with (group L, N = 21) and those without (group C, N = 21) an angiotensin type 1 (AT1) receptor antagonist (losartan). Control animals were sham operated (N = 10). Rats were sacrificed either at day 7 or day 14.

RESULTS

The degree of hydronephrosis determined morphometrically was significantly more severe in group L than group C at both day 7 and day 14, indicating that Ang II inhibition accentuated hydronephrosis. The measurement of upstream pressure within the partially ligated ureter in vivo revealed that losartan significantly attenuates the frequency of ureteral peristaltic activities. In in vitro studies using ureteral strips harvested from normal adult Sprague-Dawley rats (N = 10), Ang II (10(-8) mol/L) was shown to augment contraction, which was completely inhibited by losartan (10(-6) mol/L).

CONCLUSIONS

Ang II has a salutary effect of protecting kidneys from hydronephrosis during partial ureteral obstruction through its ability to augment ureteral peristalsis.

Pain threshold, learning and formation of brain edema in mice lacking the angiotensin II type 2 receptor

The main biological role of angiotensin II type 2 receptor (AT2) has not been established. We made use of targeted disruption of the mouse AT2 gene to examine the functional role of the AT2 receptor in the central nervous system (CNS). We have previously shown that AT2-deficient mice displayed anxiety-like behavior in comparisons with wild-type mice. In the present study, we analyzed the pain threshold, learning behavior and brain edema formation using the tail-flick test, the tail-pinch test, the passive avoidance task and cold injury, respectively. In the passive avoidance task and cold injury, no differences were found between wild-type mice and AT2-deficient mice. In contrast, the pain threshold was significantly lower in AT2-deficient mice, compared with findings in wild-type mice. The immunohistochemical distribution of beta-endorphin in the brain was analyzed quantitatively in AT2-deficient mice and wild-type mice, using microphotometry. The fluorescence intensity of beta-endorphin in the arcuate nucleus of the medial basal hypothalamus (ARC) was significantly lower in AT2-deficient mice, compared with findings in wild-type mice. We found that the AT2 receptor does not influence learning behavior and brain edema formation. As AT2-deficient mice have increased sensitivity to pain and decreased levels of brain beta-endorphin, AT2 receptors may perhaps mediate regulation of the pain threshold.

Involvement of reactive oxygen species in the activation of tyrosine kinase and extracellular signal-regulated kinase by angiotensin II

Reactive oxygen species (ROS) have been proposed to mediate vascular hypertrophy induced by angiotensin II (Ang II). Recently, we and others have shown that growth-promoting signals by Ang II involve protein tyrosine kinase (PTK) and extracellular signal-regulated kinase (ERK). However, whether ROS contribute to the Ang II-induced PTK and/or ERK activation in vascular smooth muscle cells (VSMCs) remains largely unclear. Here, we have investigated the possible involvement of ROS in Ang II-induced PTK and ERK activation. In the presence of a NADH/NADPH oxidase inhibitor, diphenyleneiodonium (DPI) or an antioxidant, alpha-tocopherol, Ang II-induced protein tyrosine phosphorylation of two major proteins (p120, p70) and ERK activation were markedly reduced, whereas ERK activation by epidermal growth factor was unaffected. DPI also inhibited Ang II-induced H2O2 production and PTK activation. In this regard, H2O2 and a membrane permeable thiol-oxidizing agent, diamide, stimulated protein tyrosine phosphorylation of p120 and p70, and ERK activation in VSMCs. H2O2 also enhanced PTK activity. From these data, we conclude that ROS play a critical role in the Ang II-induced PTK and ERK activation in VSMCs, thereby contributing to vascular growth associated with enhanced Ang II activity.

Increased gene expression of components of the renin-angiotensin system in glomeruli of genetically hypertensive rats

OBJECTIVE

The renin-angiotensin system (RAS) is implicated in the development of hypertensive glomerulosclerosis. However, no experimental evidence exists that clearly demonstrates activation of glomerular RAS in hypertensive nephropathy. We used stroke-prone spontaneously hypertensive rats (SHRSP) to examine whether RAS components are increased in glomeruli of SHRSP and whether this increase leads to an increase in mRNA levels for transforming growth factor-beta1 (TGF-beta1).

METHODS

We examined the sequential changes of urinary albumin excretion (UAE), morphology, and glomerular mRNA expression for TGF-beta1 and fibronectin (FN) in relation to glomerular mRNA expression for angiotensinogen (ATN), angiotensin converting enzyme (ACE), angiotensin II type 1a (AT1a), and type 1b (AT1b) receptors, and intervention with angiotensin II type 1 receptor antagonist candesartan and equihypotensive hydralazine.

RESULTS

In SHRSP, UAE was normal at 9 weeks of age, but became higher, beginning at 12 weeks of age, than that in the age-matched Wistar-Kyoto (WKY) rats, while SHRSP showed no glomerulosclerosis until 14 weeks of age; it was marked at 24 weeks. Plasma renin activity and plasma angiotensin II level was equivalent in the 9- and 12-week-old SHRSP and the WKY rats; both parameters, however, were elevated in 24-week-old SHRSP as compared with age-matched control. RNase protection assays showed that glomerular levels of ATN, ACE, and AT1a and AT1b receptors mRNA were significantly increased in 9-, 12-, and 14-week-old, but not in 24-week-old SHRSP, compared with age-matched WKY rats. Northern blot analysis showed that glomerular levels of TGF-beta1 and FN mRNA were higher in SHRSP than in WKY rats at all time points. Candesartan reduced UAE to control levels, whereas hydralazine reduced UAE but not to control levels. Candesartan administration for 12 weeks virtually prevented the progression of glomerulosclerosis. While candesartan reduced mRNA levels for RAS components, TGF-beta1, and FN to control levels, hydralazine was not effective in this respect. Conclusion Results suggest that increases in glomerular RAS components that occur independently of circulating RAS alter glomerular permselectivity and increase the glomerular expression of TGF-beta1 and FN in young SHRSP. Findings in old SHRSP suggest that altered glomerular permselectivity and an increased glomerular expression of TGF-beta1 and FN may be associated with the activation of systemic RAS.

International union of pharmacology. XXIII. The angiotensin II receptors

The cardiovascular and other actions of angiotensin II (Ang II) are mediated by AT(1) and AT(2) receptors, which are seven transmembrane glycoproteins with 30% sequence similarity. Most species express a single autosomal AT(1) gene, but two related AT(1A) and AT(1B) receptor genes are expressed in rodents. AT(1) receptors are predominantly coupled to G(q/11), and signal through phospholipases A, C, D, inositol phosphates, calcium channels, and a variety of serine/threonine and tyrosine kinases. Many AT(1)-induced growth responses are mediated by transactivation of growth factor receptors. The receptor binding sites for agonist and nonpeptide antagonist ligands have been defined. The latter compounds are as effective as angiotensin converting enzyme inhibitors in cardiovascular diseases but are better tolerated. The AT(2) receptor is expressed at high density during fetal development. It is much less abundant in adult tissues and is up-regulated in pathological conditions. Its signaling pathways include serine and tyrosine phosphatases, phospholipase A(2), nitric oxide, and cyclic guanosine monophosphate. The AT(2) receptor counteracts several of the growth responses initiated by the AT(1) and growth factor receptors. The AT(4) receptor specifically binds Ang IV (Ang 3-8), and is located in brain and kidney. Its signaling mechanisms are unknown, but it influences local blood flow and is associated with cognitive processes and sensory and motor functions. Although AT(1) receptors mediate most of the known actions of Ang II, the AT(2) receptor contributes to the regulation of blood pressure and renal function. The development of specific nonpeptide receptor antagonists has led to major advances in the physiology, pharmacology, and therapy of the renin-angiotensin system.

Evidence for angiotensin II type 2 receptor-mediated cardiac myocyte enlargement during in vivo pressure overload

The pathophysiological roles of the angiotensin II type 2 receptor (AT(2)) in cardiac hypertrophy remain unclear. By the targeted deletion of mouse AT(2) we were able to prevent the left ventricular hypertrophy resulting from pressure overload, while cardiac contractile functions remained normal. This implies that AT(2) is a mediator of cardiac hypertrophy in response to increased blood pressure. The effects of AT(2) deletion were independent of activation of embryonic genes for cardiac hypertrophy. However, p70(S6k), one of the key factors in cardiac hypertrophy, was markedly and specifically reduced in the ventricles of Agtr2(-)/Y mice. We propose that p70(S6k) plays a major role in AT(2)-mediated ventricular hypertrophy. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Signal transduction of angiotensin II type 1 receptor through receptor tyrosine kinase

In cultured vascular smooth muscle cells, the angiotensin II (AngII) type-1 (AT(1)) receptor generates growth-promoting signals via the epidermal growth factor (EGF) receptor system. This 'transactivation' mechanism now appears to be utilized by a variety of G-protein-coupled receptors in many cells. The AngII-induced EGF receptor transactivation leads to activation of downstream signaling molecules including Ras, ERK, c-fos, Akt/protein kinase B, and p70 S6 kinase. We propose three possible mechanisms may be involved in the transactivation, (i) an upstream tyrosine kinase, (ii) reactive oxygen species, and (iii) a juxtacrine activation of the EGF receptor ligand. Whether the EGF receptor signal transduction induced by AngII plays an essential role in cardiovascular remodeling remains to be investigated.

Long-term blood pressure telemetry in AT2 receptor-disrupted mice

OBJECTIVES

The hypertension in AT2 receptor knockout mice is imperfectly defined. Therefore, we investigated the influence of dietary salt loading and deoxycorticosterone (DOCA)-salt treatment on blood pressure and diurnal patterns of blood pressure in these mice by radiotelemetry.

METHODS

We used telemetry in AT2 receptor knockout and wild-type mice to measure blood pressure, heart rate, aortic pressure dp/dt, locomotor activity, and circadian rhythms. Salt-related effects were studied by increasing the salt in chow to 4%, adding 1% saline in drinking water, and by DOCA-salt treatment

RESULTS

Baseline blood pressures were higher in AT2 receptor knockout than in wild-type mice and were not affected by increasing the salt intake. The blood pressure increase was steeper and greater in AT2 receptor knockout than in wild-type mice after DOCA-salt treatment A circadian rhythm of blood pressure and heart rate, with higher values during the night, was seen in wild-type, but not in AT2 receptor knockout mice. In AT2 receptor knockout mice, this rhythm was only significant when daily salt intake was increased or when DOCA-salt hypertension was induced. The acrophase of blood pressure and heart rate was found between 2000 and 2400 h and was in accordance with the maximum physical activity.

CONCLUSION

These data suggest that AT2 knockout mice display slight hypertension which is not salt-sensitive. On the other hand, the susceptibility to develop DOCA-salt hypertension is increased. The study also illustrates the power of telemetry in monitoring long-term cardiovascular changes and circadian blood pressure and heart rate rhythms in genetically engineered mice.

Angiotensin II-mediated vascular smooth muscle cell growth signaling

The mechanism by which Ang II stimulates the growth of vascular smooth muscle cells was investigated by measuring the phosphorylation of mitogen-activated protein kinases ERK 1 and ERK 2. Ca2+ ionophore was found to have effects practically analogous to Ang II. We found that the signaling pathway involves the activation of epidermal growth factor receptor (EGFR) kinase, activation of the adaptor proteins Shc and Grb2, and the small G-protein Ras. Although the mechanism of AT1- (or Ca2+)-induced activation of EGFR is not yet clear, we have found that calcium-dependent protein kinase CAKss/PYK2 and c-Src are involved in this process. These studies indicate a transactivation mechanism that utilizes EGFR as a bridge between a Gq-coupled receptor and activation of phosphotyrosine generation.

Inhibition of AT1 receptor internalization by concanavalin A blocks angiotensin II-induced ERK activation in vascular smooth muscle cells. Involvement of epidermal growth factor receptor proteolysis but not AT1 receptor internalization

Recent studies of beta(2)-adrenergic receptor suggest that agonist-promoted receptor internalization may play an important role in extracellular signal-regulated kinase (ERK) activation by G protein-coupled receptors. In the present study, we explored the effects of angiotensin II (Ang II) type-1 receptor (AT(1)) internalization on Ang II-induced activation of ERK using the receptor internalization blocker concanavalin A (ConA) and the carboxyl terminus-truncated receptor mutants with impaired internalization. ConA inhibited AT(1) receptor internalization without affecting ligand binding to the receptor, Ang II-induced generation of second messengers, and activation of tyrosine kinases Src and Pyk2 in vascular smooth muscle cells (VSMC). ConA blocked ERK activation evoked by Ang II and the calcium ionophore A23187. Impairment of AT(1) receptor internalization by truncating the receptor carboxyl terminus did not affect Ang II-induced ERK activation. ConA induced proteolytic cleavage of the epidermal growth factor (EGF) receptor at carboxyl terminus and abolished Ang II-induced transactivation of the EGF receptor, which is critical for ERK activation by Ang II in VSMC. ConA also induced proteolysis of erbB-2 but not platelet-derived growth factor receptor. Thus, ConA blocks Ang II-induced ERK activation in VSMC through a distinct mechanism, the ConA-mediated proteolysis of the EGF receptor.

Association of a polymorphism at the 5'-region of the angiotensin II type 1 receptor with hypertension

Molecular variants of individual components of the renin-angiotensin system are thought to contribute to inherited predisposition towards essential hypertension. Using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and sequence analysis, we identified seven polymorphisms in the 5'-flanking region of the angiotensin II type 1 receptor (AGTR1/AT 1 ) gene. We conducted a case-control study in a sample from the Japanese population to determine whether polymorphic markers in the 5'-flanking region of the AT 1 gene were associated with essential hypertension. The study compared 149 hypertensive subjects to 156 normotensive control subjects. A significantly higher frequency of the AT 1 (-535)*T allele was observed in hypertensive subjects. Evidence was obtained that the AT 1 (-535)*T allele showed a synergistic effect on risk of hypertension with angiotensin I converting enzyme D allele (ACE*D).

PYK2/CAKbeta represents a redox-sensitive tyrosine kinase in vascular smooth muscle cells

In vascular smooth muscle cells (VSMCs), the focal adhesion kinase-related tyrosine kinase PYK2/CAKbeta is activated by vascular mitogens. Because reactive oxygen species (ROS) are assumed to mediate mitogenic signals by these agonists, we examined the possible link between ROS and PYK2 in cultured rat VSMCs. Here we present several lines of evidence showing that PYK2 is activated by ROS in VSMCs. The inhibitory effect of an antioxidant, N-acetyl-cysteine, on PYK2 activation by its specific agonists further suggests the pivotal role of PYK2 in vascular remodeling associated with enhanced ROS production.

Regulation of calcium-sensitive tyrosine kinase Pyk2 by angiotensin II in endothelial cells. Roles of Yes tyrosine kinase and tyrosine phosphatase SHP-2

Calcium-sensitive tyrosine kinase Pyk2 has been implicated in the regulation of ion channels, cellular adhesion, and mitogenic and hypertrophic reactions. In this study, we have investigated the regulation of Pyk2 by angiotensin II (Ang II) in pulmonary vein endothelial cells. We found that the Ang II-induced tyrosine phosphorylation of Pyk2, which requires the activity of Src family kinase, was specifically regulated by the Src family kinase member, Yes kinase. Moreover, we identified for the first time the constitutive association of Pyk2 with an Src homology 2 (SH2) domain-containing tyrosine phosphatase SHP-2. SHP-2 interacts with Pyk2 through a region other than its SH2 domains. Pyk2 can be dephosphorylated in vitro in SHP-2 immunoprecipitates and in intact cells expressing an NH(2) terminus-truncated form of SHP-2, which lacks the two SH2 domains but has an enhanced phosphatase activity. Ang II activates the endogenous SHP-2. Finally, the SHP-2-mediated dephosphorylation of Pyk2 correlates with the negative effect of SHP-2 on the Ang II-induced activation of extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase. Thus, the balance of Pyk2 tyrosine phosphorylation in response to Ang II is controlled by Yes kinase and by a tyrosine phosphatase SHP-2 in endothelial cells.

Renal physiology of the mouse

As the transgenic and gene-targeting technology has become an invaluable experimental approach to study the function of gene products, the need has been expanded to assess the physiology in the mouse, which is virtually the only animal species to which that new genetic technology can apply. In this regard, renal physiologists have also received fruits of success from modern technology in several key areas, and areas are expanding in both depth and scope.

The angiotensin type II receptor tonically inhibits angiotensin- converting enzyme in AT2 null mutant mice

BACKGROUND

Pharmacologic inhibition of the angiotensin-converting enzyme (ACE) limits angiotensin II (Ang II)-induced vasoconstriction and cellular proliferation. There is emerging evidence that some of the beneficial effects of ACE inhibitors may be endogenously available through the angiotensin receptor type 2 (AT2).

METHODS

To evaluate whether AT2 modulates ACE activity, we used an high-performance liquid chromatography (HPLC)-based enzymatic assay in tissues from AT2 knockout mice (Agtr2-/y) and cultured cells. These studies were complimented by physiologic studies of pharmacologic inhibition of AT2.

RESULTS

Circulating (C) and tissue ACE activities in heart (H), lung (L), and kidney (K) were doubled in Agtr2-/y mice compared with wild-type mice [162.9 +/- 17.6 mU/mL (C), 97.7 +/- 20.7 (H), 6282.1 +/- 508.3 (L), and 2295.0 +/- 87.0 (K) mU/g tissue for Agtr2-/y vs. 65.3 +/- 35.4 mU/mL (C), 44.5 +/- 8.7 (H), 3392.4 +/- 495.2 (L), and 1146.1 +/- 217.3 (K) mU/g tissue for wild-type mice, P < or = 0.05, 0.025, 0.002, and 0.0001, respectively]. Acute pharmacologic inhibition of AT2 [PD123319 (PD), 50 microg/kg/min, i. v.] significantly increased ACE activity in kidneys of wild-type mice (1591.2 +/- 104.4 vs. 1233.6 +/- 88.0 mU/g tissue in saline-infused mice, P < 0.05; P < 0.01 vs. uninfused, wild-type mice). Moreover, ACE activity increased in A10 cells exposed to PD (10-6 mol/L) together with Ang II (10-7 mol/L), but not with an AT1 antagonist (losartan, 10-6 mol/L). This heightened ACE activity appears functionally relevant because infusion of angiotensin I caused more prompt hypertension in Agtr2-/y mice than in wild-type littermates. Likewise, infusion of bradykinin, also a substrate for ACE, caused significantly less hypotension in Agtr2-/y mice than controls.

CONCLUSIONS

These studies indicate that AT2 functions to decrease ACE activity tonically, which may, in part, underlie AT2's increasingly recognized attenuation of AT1-mediated actions.

The dominant role of the prosegment of prorenin in determining the rate of activation by acid or trypsin: studies with molecular chimeras

Human prorenin activation by acid or trypsin is faster than rat prorenin by two orders of magnitude. No plausible mechanism exists to explain the difference. Two chimeric mutant prorenins were produced in CHO cells. A chimera, hPro/rRen, composed of human prorenin prosegment and rat active renin segment, was activated as fast as wild-type human prorenin at pH 3.3 and 25 degrees C or by trypsin (1 microg/ml). The other chimera, rPro/hRen, composed of rat prorenin prosegment and human active renin segment, was activated as slowly as wild-type rat prorenin at pH 3.3 and 25 degrees C or by trypsin (50 microg/ml). These results indicate that the rate of activation of prorenin is predominantly determined by the N-terminal pro-sequence. Plausible mechanisms are discussed.

Induction of cyclooxygenase-2 by angiotensin II in cultured rat vascular smooth muscle cells

Angiotensin II (Ang II) stimulates the release of prostaglandins (PGs) in various cells and tissues. Recently, cyclooxygenase-2 (COX-2) emerged as a new key regulator for PG synthesis. In the present study, we investigated whether Ang II regulates COX-2 expression in cultured rat vascular smooth muscle cells (VSMCs). Ang II markedly increased the expression of COX-2 mRNA in a time- and dose-dependent manner. This effect was completely blocked by the Ang II type 1 receptor antagonist losartan but not by the Ang II type 2 receptor antagonist PD123319. The p42/44 mitogen-activated protein kinase (MAPK) kinase-1 inhibitor PD98059 and the p38 MAPK inhibitor SB203580 significantly suppressed Ang II-induced COX-2 mRNA and protein expression. Ang II did not increase transcription of the COX-2 gene, as examined with a COX-2 promoter/luciferase chimeric plasmid construct. Instead, it suppressed the degradation of COX-2 mRNA. PD98059 and SB203580 markedly enhanced the decay of COX-2 mRNA induced by Ang II, implying that p42/44 and p38 MAPK activated by Ang II play a role in the regulation of COX-2 through stabilization of its mRNA. The COX-2-specific inhibitor NS-398 attenuated Ang II-stimulated DNA and protein synthesis, as well as PGE(2) production by VSMCs. These results suggest that Ang II regulates COX-2 expression and PG production and modulates cell proliferation through MAPK-mediated signaling pathways in rat VSMCs.

Inhibition of pressure natriuresis in mice lacking the AT2 receptor

Inhibition of pressure natriuresis in mice lacking the AT2 receptor.

BACKGROUND

Angiotensin II type 2 (AT2) receptor knockout mice have higher blood pressures than wild-type mice; however, the hypertension is imperfectly defined. We tested the hypothesis that renal mechanisms could be contributory.

METHODS

We conducted pressure-natriuresis-diuresis experiments, measured renal cortical and medullary blood flow by laser Doppler methods, and explored cytochrome P450-dependent arachidonic acid metabolism by means of reverse transcription-polymerase chain reaction.

RESULTS

Blood pressure was 15 mm Hg higher in AT2 receptor knockout mice than in controls, and pressure diuresis and natriuresis curves were shifted rightward. At similar renal perfusion pressures (113 to 118 mm Hg), wild-type mice excreted threefold more sodium and water than AT2 receptor knockout mice. Fractional sodium and water excretion curves were shifted rightward in parallel. Renal blood flow ranged between 6.72 and 7.88 mL/min/g kidney wet weight (kwt) in wild-type and between 5.84 and 6.15 mL/min/g kwt in AT2 receptor knockout mice. Renal vascular resistance was increased in AT2A receptor knockout mice. Cortical blood flow readings leveled at 2.5 V in wild-type and 1.5 V in AT2 receptor knockout mice. Medullary blood flow readings ranged between 0.8 and 1.0 V and increased 116% in wild-type mice as renal perfusion pressure was increased. This increase did not occur in AT2 receptor knockout mice. The glomerular filtration rate (GFR) was similar in both groups at approximately 1 mL/min/g kwt. Renal microsomes from AT2 receptor knockout mice had less activity in hydroxylating arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-meter) than controls, whereas renal AT1 receptor gene expression was increased in AT2 receptor knockout mice.

CONCLUSIONS

Hemodynamic and tubular factors modify renal sodium handling in AT2 receptor knockout mice and may cause hypertension. AT2 receptor disruption induces alterations of other regulatory systems, including altered arachidonic acid metabolism, that may contribute to the intrarenal differences observed between AT2 receptor knockout and wild-type mice.

Involvement of Rho-kinase in angiotensin II-induced hypertrophy of rat vascular smooth muscle cells

Angiotensin II (Ang II) is now believed to play a critical role in the pathogenesis of hypertrophy and/or hyperplasia of vascular smooth muscle cells (VSMCs). Several G(i)- and G(q)-coupled receptors, including the Ang II type 1 (AT(1)) receptor, activate Rho and Rho-associated kinase in Swiss 3T3 cells and cardiac myocytes. However, little is known about the role of Rho-kinase in Ang II-induced vascular hypertrophy in VSMCs. In the present study, we explored the role of Rho and Rho-kinase in Ang II-induced protein synthesis in VSMCs. In unstimulated cells, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II (100 nmol/L). This effect was completely blocked by the AT(1) receptor blocker candesartan but not by the Ang II type 2 (AT(2)) receptor antagonist PD123319. Botulinum C(3) exoenzyme, which inactivated RhoA, attenuated Ang II-induced [(3)H]leucine incorporation. The specific Rho-kinase inhibitor, Y-27632, dose-dependently abolished Ang II-induced protein synthesis and also suppressed Ang II-induced c-fos mRNA expression. On the other hand, Y-27632 had no effect on Ang II-stimulated phosphorylation of p70 S6 kinase and extracellular signal-regulated kinase 1/2, which are reported to be involved in Ang II-induced protein synthesis, nor had it any effect on the Ang II-induced phosphorylation of PHAS-I, a heat- and acid-stable eIF-4E-binding protein. The phosphorylation of PHAS-I is regulating for translation initiation. These observations suggest that the Rho, Rho-kinase, and c-fos pathways may play a role in Ang II-induced hypertrophic changes of VSMCs through a novel pathway.

Intracellular signaling of angiotensin II-induced p70 S6 kinase phosphorylation at Ser(411) in vascular smooth muscle cells. Possible requirement of epidermal growth factor receptor, Ras, extracellular signal-regulated kinase, and Akt

Activation of p70 S6 kinase (p70(S6K)) by growth factors requires multiple signal inputs involving phosphoinositide 3-kinase (PI3K), its effector Akt, and an unidentified kinase that phosphorylates Ser/Thr residues (Ser(411), Ser(418), Ser(424), and Thr(421)) clustered at its autoinhibitory domain. However, the mechanism by which G protein-coupled receptors activate p70(S6K) remains largely uncertain. By using vascular smooth muscle cells in which we have demonstrated Ras/extracellular signal-regulated kinase (ERK) activation through Ca(2+)-dependent, epidermal growth factor (EGF) receptor transactivation by G(q)-coupled angiotensin II (Ang II) receptor, we present a unique cross-talk required for Ser(411) phosphorylation of p70(S6K) by Ang II. Both p70(S6K) Ser(411) and Akt Ser(473) phosphorylation by Ang II appear to involve EGF receptor transactivation and were inhibited by dominant-negative Ras, whereas the phosphorylation of p70(S6K) and ERK but not Akt was sensitive to the MEK inhibitor. By contrast, the phosphorylation of p70(S6K) and Akt but not ERK was sensitive to PI3K inhibitors. Similar inhibitory pattern on these phosphorylation sites by EGF but not insulin was observed. Taken together with the inhibition of Ang II-induced p70(S6K) activation by dominant-negative Ras and the MEK inhibitor, we conclude that Ang II-initiated activation of p70(S6K) requires both ERK cascade and PI3K/Akt cascade that bifurcate at the point of EGF receptor-dependent Ras activation.

Elements of a paracrine tubular renin-angiotensin system along the entire nephron

The renin-angiotensin system is a major regulator of body sodium, predominantly through the actions of intrarenal angiotensin II of unclear origin. We show that polarized epithelium of the proximal tubule synthesizes and secretes angiotensinogen at its apical side and that the protein can be detected in urine as a function of dietary sodium. Furthermore, we demonstrate that renin is expressed and secreted in a restricted nephron segment, the connecting tubule, also in a sodium-dependent fashion. A paracrine renin-angiotensin system operating along the entire nephron may contribute to long-term arterial pressure regulation by integrating distant tubular sodium-reabsorbing functions.

Role of the angiotensin II type-2 receptor in the mouse central nervous system

There are two known major angiotensin II receptor subtypes, type 1 (AT1) and type 2 (AT2), both of which are present in the brain. AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin II-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the central nervous system, angiotensin II has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. While the role of the AT2 receptor in the brain is less well understood, recent knockout studies point to their involvement in behavioral and cardiovascular functions. We discuss here evidence regarding the function of the AT2 receptor in the brain, determined using mice lacking the AT2 receptor.

Potent antihypertrophic effect of the bradykinin B2 receptor system on the renal vasculature

BACKGROUND

Angiotensin type 1 (AT1) receptor-deficient mice (Agtr1-/-), which selectively lack both AT1A and AT1B receptor genes, are characterized by marked intrarenal vascular thickening. In the present study, we explored the possible involvement of the kinin-kallikrein system in the development of this renal vascular hypertrophy.

METHODS

Wild-type and Agtr1-/- mice were examined for the developmental regulation pattern of the kinin-kallikrein system and treated with aprotinin (a kallikrein inhibitor), AcLys [D-b Nal7, Ile8] des-Arg9-bradykinin (a bradykinin B1 receptor antagonist), or Hoe-140 (a bradykinin B2 receptor antagonist) from 3 to 14 days of age.

RESULTS

The normal postnatal up-regulation of kininase II was organ-specifically suppressed in Agtr1-/- kidneys at 2 and 3 weeks of age. Immunohistochemical staining in Agtr1-/- mice revealed tissue kallikrein staining along the nephron from connecting tubules to cortical collecting tubules in proximity to the hypertrophic vasculature, whereas tissue kallikrein staining was confined to connecting tubules in wild-type mice. Aprotinin and Hoe-140 accelerated the vascular hypertrophy significantly as determined by wall thickness ratio, whereas B1 receptor antagonism had no effect.

CONCLUSION

The kinin-kallikrein system in the Agtr1-/- mouse kidney is functionally activated by local suppression of kininase II and extensive redistribution of kallikrein to perivascular areas. This activation, specific to the kidney, serves to dampen a development of the marked vascular hypertrophy. These results demonstrate, to our knowledge for the first time, the antihypertrophic effect of the bradykinin B2 receptor system on the renal vasculature in vivo.

Intracellular signaling in rat cultured vascular smooth muscle cells: roles of nuclear factor-kappaB and p38 mitogen-activated protein kinase on tumor necrosis factor-alpha production

Lipopolysaccharide (LPS) is responsible for initiating host responses leading to septic shock, and tumor necrosis factor-alpha (TNF alpha) is thought to be its primary mediator. In addition, TNF alpha is one of the major components of the pathogenesis of insulin resistance in various conditions. It has been shown that LPS induced TNF alpha production in rat vascular smooth muscle cells (VSMC). However, little is known about the signaling pathway by which VSMC in culture produce TNF alpha. We investigated the possible signaling components involved in this pathway. LPS elicited phosphorylation of p42/44 mitogen-activated protein kinase (MAPK) and p38 MAPK, degradation of inhibitor of kappaB (IkappaB), and an increase in nuclear binding activity of activating protein-1 and nuclear factor-kappaB (NF-kappaB). Different types of NF-kappaB inhibitors, pyrrolidine dithiocarbamate and MG132, which specifically abolished IkappaB degradation and subsequent NF-kappaB activation by LPS, suppressed TNF alpha secretion from VSMC. Although PD98059, a specific MAPK kinase inhibitor and SB203580, a specific p38 MAPK inhibitor, had no effect on NF-kappaB activity, SB203580 suppressed TNF alpha secretion; however, PD98059 did not. A cotransfection assay showed that transfection of dominant negative IkappaB or pretreatment with SB203580 suppressed the TNF alpha gene promotor-dependent transcription. TNF alpha messenger RNA expression induced by LPS was inhibited by pyrrolidine dithiocarbamate, MG132, and SB203580, but not by PD98059. These observations indicate that TNF alpha production in VSMC is stimulated by LPS, and its transcription and translation are dependent on NF-kappaB activation through proteasome-mediated IkappaB degradation. It is likely that p38 MAPK may play a critical role in regulating transcription of the TNF alpha gene in VSMC, unlike in other cell lines.

Increased renal vasodilator prostanoids prevent hypertension in mice lacking the angiotensin subtype-2 receptor

The angiotensin subtype-1 (AT(1)) receptor mediates renal prostaglandin E(2) (PGE(2)) production, and pharmacological blockade of the angiotensin subtype-2 (AT(2)) receptor potentiates the action of angiotensin II (Ang II) to increase PGE(2) levels. We investigated the role of the AT(2) receptor in prostaglandin metabolism in mice with targeted deletion of the AT(2) receptor gene. Mice lacking the AT(2) receptor (AT(2)-null) had normal blood pressure that was slightly elevated compared with that of wild-type (WT) control mice. AT(2)-null mice had higher renal interstitial fluid (RIF) 6-keto-PGF(1alpha) (a stable hydrolysis product of prostacyclin [PGI(2)]) and PGE(2) levels than did WT mice, and had similar increases in PGE(2) and 6-keto-PGF(1alpha) in response to dietary sodium restriction and Ang II infusion. In contrast, AT(2)-null mice had lower PGF(2alpha) levels compared with WT mice during basal conditions and in response to dietary sodium restriction or infusion of Ang II. RIF cAMP was markedly higher in AT(2)-null mice than in WT mice, both during basal conditions and during sodium restriction or Ang II infusion. AT(1) receptor blockade with losartan decreased PGE(2), PGI(2), and cAMP to levels observed in WT mice. To determine whether increased vasodilator prostanoids prevented hypertension in AT(2)-null mice, we treated AT(2)-null and WT mice with indomethacin for 14 days. PGI(2), PGE(2), and cAMP were markedly decreased in both WT and AT(2)-null mice. Blood pressure increased to hypertensive levels in AT(2)-null mice but was unchanged in WT. These results demonstrate that in the absence of the AT(2) receptor, increased vasodilator prostanoids protect against the development of hypertension.

Mechanotransduction of rat aortic vascular smooth muscle cells requires RhoA and intact actin filaments

The growth-promoting effect of mechanical stress on vascular smooth muscle cells (VSMCs) has been implicated in the progress of vascular disease in hypertension. Extracellular signal-regulated kinases (ERKs) have been implicated in cellular responses, such as vascular remodeling, induced by mechanical stretch. However, it remains to be determined how mechanical stretch activates ERKs. The cytoskeleton seems the most likely candidate for force transmission into the interior of the cell. Therefore, we examined (1) whether the cytoskeleton involves mechanical stretch-induced signaling, (2) whether Rho is activated by stretch, and (3) whether Rho mediates the stretch-induced signaling in rat cultured VSMCs. Mechanical stretch activated ERKs, with a peak response observed at 20 minutes, followed by a significant increase in DNA synthesis. Treatment with the ERK kinase-1 inhibitor, PD98059, inhibited the stretch-induced increase in DNA synthesis. Cytochalasin D, which selectively disrupts the network of actin filaments, markedly inhibited stretch-induced ERK activation. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to mechanical stretch. Botulinum C3 exoenzyme, which inactivates Rho p21 (known to participate in the reorganization of the actin cytoskeleton), attenuated stretch-induced ERK activation. Inhibition of Rho kinase (p160ROCK) also suppressed stretch-induced ERK activation dose dependently. Our results suggest that mechanotransduction in VSMCs is dependent on intact actin filaments, that Rho is activated by stretch, and that Rho/p160ROCK mediates stretch-induced ERK activation and vascular hyperplasia.

Epidermal growth factor receptor is indispensable for c-Fos expression and protein synthesis by angiotensin II

We have reported that angiotensin II induces the epidermal growth factor (EGF) receptor transactivation leading to extracellular signal-regulated kinase (ERK) activation in rat vascular smooth muscle cells. Here, we report that the EGF receptor kinase inhibitor AG1478 and the ERK kinase inhibitor PD98059 markedly inhibited angiotensin II-induced c-Fos expression and protein synthesis but not c-Jun expression in these cells. These data suggest that the EGF receptor transactivation and subsequent ERK activation are indispensable for angiotensin II-mediated growth promotion of vascular smooth muscle cells providing a new mechanistic insight whereby angiotensin II contributes abnormal vascular remodeling.

Angiotensin II type 1 receptor expression in two cases of juxtaglomerular cell tumor: correlation to negative feedback of renin secretion by angiotensin II

The angiotensin II (Ang II) type 1 (AT1) receptor is highly expressed on juxtaglomerular (G) cells and is assumed to be involved in the negative short loop feedback regulation of renin secretion and in the suppression of Ang II-mediated JG cell proliferation and/or growth. However, as JG cell tumor is rare, expression and pathophysiological significance of AT1 receptor expression in JG cell tumor remain unknown. In the present study, we investigated renin responses to various treatments, including the angiotensin converting enzyme inhibitor captopril, and correlated the results with AT1 and Ang II type 2 (AT2) receptor mRNA expression levels in two cases of JG cell tumor. Whereas plasma renin activity (PRA) did not show any significant change in Case 1, it was increased by 72% in Case 2 in response to captopril challenge. In concordance with these results, AT1 receptor mRNA was not detected in tumor tissue of Case 1 but was clearly demonstrated in the tumor of Case 2. AT2 receptor mRNA expression was not detected in either of the cases. In contrast to captopril challenge, PRA was suppressed by 30% in Case 1 and 42% in Case 2 in response to saline infusion, and was increased by 230% in Case 1 and 59% in Case 2 in response to furosemide-upright posture for 2 h. These results suggest that the short loop feedback inhibition of renin secretion by Ang II in JG cell tumor is closely related to AT1 receptor expression levels in the tumor tissue. In addition, the result suggested that despite its autonomy, renin secretion from JG cell tumor is still under physiological regulatory control.

Angiotensin receptors: molecular biology and signalling

1. The active peptide hormone angiotensin II (AngII) is formed from its prohormone angiotensinogen by way of inactive angiotensin I. The highly specific protease, renin, responsible for the initiation of this system was elusive and considered unstable. We isolated it in a pure and stable form from the kidney of the pig, human, rat, and land submandibular glands of the mouse. It was shown that there is only one type of renin with highly stringent substrate specificity, except certain strains of the mouse which have two gene products. 2. The well-known diversity of action of AngII can be attributed to the presence of more than two subtypes, AT1 and AT2, as well as multiple signalling pathways for both of them. 3. The first subtype AT1 was shown to mediate most of the traditionally recognized AngII functions such as vasoconstriction, electrolyte homeostasis etc. 4. Although the identification of the signalling modes of the second subtype AT2 still remains elusive, we and others have shown evidence that its action is generally antagonistic to that of AT1. AT2 inhibits AT1 (growth factor-stimulated cell growth), AT2 attenuates the vasoconstriction induced by AT1. Since AT2 seems to mediate nitric oxide formation in the renal cells, it may initiate a natriuretic pathway in contrast to the sodium-retaining action of AT1-mediated AngII action. 5. Newer mechanisms and functions of these and other receptors will be clarified by the combination of molecular, cellular and integrated physiological studies.

Sustained hypersensitivity to angiotensin II and its mechanism in mice lacking the subtype-2 (AT2) angiotensin receptor

The vast majority of the known biological effects of the renin-angiotensin system are mediated by the type-1 (AT1) receptor, and the functions of the type-2 (AT2) receptor are largely unknown. We investigated the role of the AT2 receptor in the vascular and renal responses to physiological increases in angiotensin II (ANG II) in mice with targeted deletion of the AT2 receptor gene. Mice lacking the AT2 receptor (AT2-null mice) had slightly elevated systolic blood pressure (SBP) compared with that of wild-type (WT) control mice (P < 0.0001). In AT2-null mice, infusion of ANG II (4 pmol/kg/min) for 7 days produced a marked and sustained increase in SBP [from 116 +/- 0.5 to 208 +/- 1 mmHg (P < 0.0001) (1 mmHg = 133 Pa)] and reduction in urinary sodium excretion (UNaV) [from 0.6 +/- 0.01 to 0.05 +/- 0.002 mM/day (P < 0.0001)] whereas neither SBP nor UNaV changed in WT mice. AT2-null mice had low basal levels of renal interstitial fluid bradykinin (BK), and cyclic guanosine 3', 5'-monophosphate, an index of nitric oxide production, compared with WT mice. In WT mice, dietary sodium restriction or ANG II infusion increased renal interstitial fluid BK, and cyclic guanosine 3', 5'-monophosphate by approximately 4-fold (P < 0.0001) whereas no changes were observed in AT2-null mice. These results demonstrate that the AT2 receptor is necessary for normal physiological responses of BK and nitric oxide to ANG II. Absence of the AT2 receptor leads to vascular and renal hypersensitivity to ANG II, including sustained antinatriuresis and hypertension. These results strongly suggest that the AT2 receptor plays a counterregulatory protective role mediated via BK and nitric oxide against the antinatriuretic and pressor actions of ANG II.

Communication between myocytes and fibroblasts in cardiac remodeling in angiotensin chimeric mice

To characterize the mode of action of angiotensin II (Ang II) in cardiac remodeling, we generated chimeric mice that are made of both homozygous Ang II receptor type 1A gene (Agtr1a) null mutant cells and Agtr1a intact cells expressing the lacZ gene (ROSA26). Both Agtr1a null and intact myocytes and interstitial cells independently form areas that are randomly distributed throughout the heart. The distribution of ROSA26 cardiomyocytes overlaps completely with that of Ang II binding, indicating that the majority of Ang II receptors reside on cardiomyocytes. When Ang II (1 ng/g body weight/min) was infused for 2 weeks, mice developed mild to moderate hypertension. The proliferating cardiac fibroblasts identified by bromodeoxyuridine staining were present predominantly in the areas surrounded by Agtr1a intact cardiomyocytes. When control chimeric mice made of wild-type cells and ROSA26 cells (i.e., both carrying intact Agtr1a) were infused with Ang II, fibroblast proliferation was found equally in these cardiomyocyte types. When compared with Agtr1a null mutant chimeras, the control chimeras had more extensive cardiac fibrosis, most prominently in perivascular regions. Therefore, in response to Ang II, cardiac fibroblasts proliferate through both the local and systemic action of Ang II. Importantly, the former is determined by the Ang II receptor of neighboring cardiomyocytes, indicating that a communication between myocytes and fibroblasts plays an important role during Ang II-dependent cardiac remodeling.

Fyn kinase-directed activation of SH2 domain-containing protein-tyrosine phosphatase SHP-2 by Gi protein-coupled receptors in Madin-Darby canine kidney cells

SHP-2, an SH2 domain-containing protein-tyrosine phosphatase, plays an important role in receptor tyrosine kinase-regulated cell proliferation and differentiation. Little is known about the activation mechanisms and the participation of SHP-2 in the activity of G protein-coupled receptors lacking intrinsic tyrosine kinase activity. We show that the activity of SHP-2 (but not SHP-1) is specifically stimulated by the selective alpha2A-adrenergic receptor agonist UK14304 and by lysophosphatidic acid (LPA) in Madin-Darby canine kidney (MDCK) cells. UK14304 and LPA promote the tyrosine phosphorylation of SHP-2 and its association with Grb2. The agonist-induced direct interaction of Grb2 with SHP-2 is mediated by the SH2 domain of Grb2 and the tyrosine phosphorylation of SHP-2. Rapid activation of Src family kinase by UK14304 preceded the SHP-2 activation. Among the Src family members (Src, Fyn, Lck, Yes, and Lyn) present in MDCK cells, Fyn was the only one specifically associated with SHP-2, and the physical interaction between them, which requires the Src family kinase activity, was increased in response to the agonists. Pertussis toxin, PP1 (a selective Src family kinase inhibitor), or overexpression of a catalytically inactive mutant of Fyn blocked the UK14304- or LPA-stimulated activity of SHP-2, SHP-2 tyrosine phosphorylation, and SHP-2 association with Grb2. Therefore, we have demonstrated for the first time that the activation of SHP-2 by these Gi protein-coupled receptors requires Fyn kinase and that there is a specific physical interaction of Fyn kinase with SHP-2 in MDCK cells.

Vascular response to angiotensin II is exaggerated through an upregulation of AT1 receptor in AT2 knockout mice

Blood pressure is elevated and pressor response to angiotensin II (Ang II) is exaggerated in AT2 null mice. The purpose of the present study was to elucidate the mechanism for the increased responsiveness to Ang II in the mice. The contraction of aortic strips generated by Ang II was significantly greater in the AT2 gene-deleted mice than the control, which was completely abolished by AT1 antagonist losartan. The aortic content of AT1 receptor was significantly increased (P < 0.05, n = 5) in the AT2 null mice (212 +/- 58.2 fmol/mg protein) compared with the control (98.2 +/- 55.9 fmol/mg protein). While both AT1 and AT2 mRNAs were expressed in the aorta of the control mice, only AT1 mRNA was expressed in the AT2 knockout mice. The expression of AT1 mRNA in the AT2 knockout mice was significantly higher (1.5-fold, P < 0.05, n = 5) than that in the control. The present study clearly demonstrated that the increased vascular reactivity to Ang II in AT2 knockout mice is at least partly due to an increased vascular AT1 receptor expression and suggested that AT2 counteracts AT1-mediated vascular action of Ang II through downregulation of AT1 receptor by a crosstalk between these receptors by some as yet unknown mechanisms.